Velocity of CO2 exchanges in the lungs.

As outlined above, investigations over the past decade have provided further insight into the kinetics of many of the component processes that affect the overall velocity of CO2 exchanges in the lungs. The evolution of our understanding of the importance and role of these reactions and transport processes has not been entirely predictable. A variety of investigations into Roughton's original hypothesis regarding the mechanism of pH equilibration in capillary blood resulted in a renewed interest in carbonic anhydrase in the lung and other tissues. These latter studies have helped better define the location and kinetic properties of lung CA and its role in the overall velocity of CO2 exchange. Concurrently, a wealth of information has emerged regarding the transport properties of the red cell membrane. This has led to a better understanding of the mechanisms and characteristics of the band 3-mediated pathway for electroneutral anion exchange. Unfortunately, most of the kinetic data for this pathway have been obtained under nonphysiological conditions, making it difficult to utilize them directly in analyses of lung CO2 exchange kinetics. The potential detrimental effect of pharmacological agents in modifying lung CA and red cell CA activity and red cell anion exchange kinetics is among the more important factors to have emerged in the past decade. Ironically, the original question of whether a significant blood pH disequilibrium is present in the arterial circulation in man in vivo remains unresolved. However, the mechanisms underlying these phenomena are now recognized to be more complex than originally appreciated.